Limonin thresholds

Influence of Sweeteners on Bitterness. In model system studies, natural fruit juice sugars were observed to raise the limonin threshold (24). An expanded study of natural and artificial sweeteners (26) demonstrated that sucrose, neohesperidin dihydro-chalcone (NHD), hesperetin dihydrochalcone glucoside (HDG) and aspartylphenylalanine methyl ester (AP) all raise the limonin threshold. At low sweetness levels HDG was the most effective followed by AP and NHD. Sucrose was without effect up to the 2% level. At sweetness levels equivalent to 1% sucrose, HDG, AP and NHD raised the limonin threshold in water from 1.0 ppm to 3.2, 2.5 and 1.3 ppm, respectively. Because of its high sweetness intensity, the concentration of NHD (16 ppm) was considerably lower than HDG (80 ppm) and AP (90 ppm). At 3-10% sucrose sweetness equivalency, the effectiveness of NHD increased substantially, sucrose moderately and HDG slightly, while that of AP decreased. Therefore, the sweeteners HDG, AP and NHD can effectively suppress limonin bitterness at low concentrations. 

Another important interaction is that of limonin with the bitter flavanone glycoside naringin. Both of these bitter substances are present in grapefruit juice and Guadagni et al. (25) found that they interact at subthreshold levels in an additive way. Less than threshold amounts of limonin or naringin contribute to the bitterness of a mixture of the two compounds. The bitterness of the mixture can be predicted by adding the taste-unit contribution of each component (taste unit = concentration/ threshold). 

Chalcones and Pihydrochalcones. Chalcones and dihydro-chalcones are Intensely sweet compounds (39) that are effective in raising the threshold at which the bitterness of naringin and limonin is perceived (46). As illustrated in Figure 5, chalcones are easily formed fromTlavanone glycosides by the addition of alkali and dihydrochalcones are formed from hydrogenated chalcones. Like the flavanone neohesperidosides, the chalcones and dihydrochalcones vary in the intensity of their taste response. 

None of the methods developed thus far can be considered ideal, especially for routine quality control purposes. Among their disadvantages are subjective readout methods, time-consuming separations, questionable specificity, and limited applicability. Recent work showing that taste thresholds for limonin are lower than had previously been assumed for significant proportions of the population emphasizes the need for a better assay method. The use of a protein specific for limonin, such as an enzyme or antibody, could provide the basis for an improved assay." (9)... 

Limonin and nomilin are two bitter limonoids present in citrus juices. Systematic organoleptic tests showed that the bitterness threshold is 6 ppm (5) for limonin, and 6 ppm (6) or 3 ppm (7) for nomilin. The bitterness due to limonin develops gradually in juices after extraction from certain varieties of oranges, grapefruit, lemon, Natsudaidai, mandarin and some other minor citrus such as Iyokan and Ponkan. This phenomenon is generally referred to as "delayed bitterness". 

Cellulose acetate jels and beads were used to effectively remove limonin below the bitterness threshold from citrus juies (43). The resins can be regenerated readily by washing with a small volume of warm water. The process has been used commercially in Australia in debittering bitter orange juices (47). 

Limonin bitterness is especially acute in juice obtained from short maturation time fruits such as navel oranges. Juices obtained from most of the other citrus crops, such as Valencia oranges, usually do not have the limonin bitterness problem, as the concentration of limonin has decreased sufficiently by the time the processing season starts. Juices from navel oranges, particularly early- to mid-season fruits, can contain over 30 ppm of limonin, which is well above the bitterness detection threshold of 6 ppm. Grapefruit also has significant levels of limonin, which vary from 10 ppm in the early season to 1 ppm or less in the late season. 

Nomilin, a limonoid that is approximately twice as bitter as limonin (Hashinaga et al. 1977), was found in various citrus species. Nomilin concentrations ranging from 1.6 to less than 0.1 ppm were found in commercial grapefruit juices with the greatest concentration found in early-season juices (Rouseff 1982). The concentrations in the early season exceed the detection threshold of 0.8 ppm but they are below the recognition threshold of 2.1 ppm (Rouseff and Matthews 1984). Other bitter limonoids are also present in the citrus juices, but at much lower concentrations than limonin. They do not contribute significantly to the bitterness problem. 

Guadagni, D.G., Maier, V.P., Tumbaugh, J.G. Effect of some citms juice constituents on taste thresholds for limonine and naringin bitterness. J. Sci. Food Agric. 24, 1277 (1973)... 

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